A new uplift episode at Campi Flegrei caldera (Southern Italy): implications for unrest interpretation and eruption hazard evaluation.

Campi Flegrei caldera, which includes the highly urbanized city of Naples, is the most dangerous volcanic area in the world as it represents a serious threat to around two million people. During the last four decades it has experienced a huge uplift phase, which reached about 3.5 m in 1985 when a subsidence phase started. Recent geodetic data demonstrates that such a subsidence phase has terminated, and a new uplift episode started in November 2004 has so far resulted in about 0.04 m of uplift (October 2006). Here we show that the present episode, which appears slower but longer than previous small uplifts, sheds light both on the origin of small and large unrests, and on the conditions to evolve toward large unrests. Our simple but robust method is based on continuous GPS measurements and compares the ratio between maximum horizontal to vertical displacements. Results show that such unrests are due to overpressure in a deeper source of fluids of magmatic origin. When the resulting stress increase causes fracturing of the overlying rocks, magmatic fluids are injected in shallower aquifers and generate large uplifts. This means that, given the common magmatic origin, both small and large unrests contribute to the increase in pressure of the magma chamber which, given sufficient magnitude, can cause fracturing of the overburden and produce an eruption. In this paper, we show that the maximum horizontal to vertical displacement ratio can be a powerful indicator of source changes, and can give important information for volcanic eruption forecast.